Refactor: Standardize Playwright route pattern convention in E2E fixture helpers

## Summary

Fixture helpers under `browser_tests/fixtures/helpers/` currently use a mixed convention for Playwright route patterns:

- Some fixtures (e.g. the assets and prompt handlers) anchor routes with regex patterns such as `/\/api\/assets(?:\?.*)?$/` and `/\/api\/prompt$/`.
- The `TemplateHelper.mockIndex()` handler added in #12999 uses a glob pattern (`**/api/workflow_templates`) consistent with the `README`-documented `**/api/object_info` style.

Both approaches work correctly, but the inconsistency makes the fixture layer harder to maintain and reason about. A follow-up refactor pass should align all route patterns to a single convention (either regex or glob) across the fixture helpers.

## References

- PR: #12999
- Comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/12999#discussion_r3443589817
- Requested by: @jaeone94